The present invention relates to a composite material comprising a metal matrix reinforced with particles of a reinforcing material and a process for manufacturing such a composite material.
Composites comprising metal alloys reinforced with hard particles such a silicon carbide are known in the art. Composites comprising aluminum alloys reinforced with hard particles are particularly well known in the art. The latter have been used in a wide variety of applications including pistons for automotive engines and drive shafts. Aluminum metal and alloys reinforced with a particulate such as silicon carbide, aluminum oxide or aluminum nitride is a particularly attractive material because of highly attractive properties such as higher elastic modulus than aluminum, a density similar to aluminum, good thermal conductivity, low thermal expansion and good tensile properties.
Commercial efforts to make a reinforced aluminum material have included liquid metal processes and powder metallurgy processes. The liquid metal processes, such as stirring particulate into molten aluminum and casting a shape, suffer from several disadvantages. The volume fraction of particulate is generally limited to less than about 30 percent in this type process because the mixture becomes too viscous to mix. The reaction between many liquid aluminum alloys and silicon carbide reinforcement materials can result in the formation of aluminum carbide, which tends to degrade composite properties.
Powder metallurgy processes offer a way of making much higher volume fraction composites, up to about 70 percent particulates, and avoid the problem of chemical reactivity. In the simplest such process, a metal alloy powder and a particulate powder are mixed, then consolidated by compacting at an elevated temperature. This process has the primary disadvantage of inhomogeneous particulate distribution. Powder metallurgy processes may also have problems such as oxidation of the metal alloy powder, residual gas entrapment, and the low green strength or as-compacted strength of higher volume fraction particulates.
An alternative to simple blending of metal alloy powder and particulate powder comprises mechanical alloying wherein the matrix metal material and reinforcing particles are subjected to energetic mechanical milling. The milling causes the metallic matrix material to enfold around each of the reinforcing particles while the charge being subjected to energetic milling is maintained in a powdery state. This type of milling provides a strong bond between the matrix material and the surface of the reinforcing particle. After the milling is completed, the resulting powder is consolidated or compacted and subjected to working such as rolling, sinter forging, cold isostatic pressing and hot forging, hot pressing or cold isostatic pressing and hot extrusion. Aside from the relatively high cost of milling, this method also has the disadvantage of inhomogeneous particulate distribution.
What are desired are processes for fabricating metal matrix composites consisting of discontinuous reinforcing particles in a metal matrix which overcome these disadvantages.
Accordingly, it is an object of the present invention to provide novel processes for fabricating metal matrix composites consisting of discontinuous reinforcing particles in a metal matrix.
Other objects and advantages of the present invention will be apparent to those skilled in the art.